A communication system generally comprises at least one transmitter and at least one receiver. The communication system may be, for example, a cellular radiotelephone system having at least one base station, including a transmitter, and a plurality of subscriber radiotelephones, each including a receiver. The transmitter typically communicates signals to the receiver over a communication channel. When a transmitter is not communicating with a receiver, the receiver may operate in either a continuous receive mode or a discontinuous receive mode.
In the continuous receive mode of operation, the receiver continuously receives and decodes all signals sent by the transmitter until the receiver recognizes a signal intended for the receiver. A recognized signal may be a unique telephone number, for example. After the receiver recognizes the signal intended for the receiver, the receiver is then permitted to receive other signals sent by the transmitter over the communication channel, such as a telephone call, for example. The continuous receive operating mode is sometimes known as "stand-by mode".
In the discontinuous receive mode of operation, the receiver selectively receives and decodes signals sent by the transmitter until the receiver recognizes a signal intended for the receiver. After the receiver recognizes the signal intended for the receiver, the receiver is then permitted to receive other signals sent by the transmitter over the communication channel, such as a telephone call, for example. The discontinuous receive mode of operation is also sometimes called "intermittent receive mode". The receiver is able to selectively receive and decode signals sent by the transmitter by taking advantage of a predetermined communication relationship between the transmitter and the receiver. Such a predetermined communication relationship may be, for example, an intermittent signaling scheme used by the transmitter. The intermittent signaling scheme may be, for example, a time division multiple access (TDMA) signaling scheme used by conventional digital cellular radiotelephone systems. The intermittent signaling scheme permits the receiver to turn off its receive and decode functions during the times when the transmitter will not send a signal to the receiver, and to turn on its receive and decode functions during the times when the transmitter will send a signal to the receiver.
A distinction between receivers operating in continuous receive mode and receivers operating in discontinuous receive mode is the amount of current drain on the power supply of the receiver. When a receiver is portable such as in a portable radiotelephone, for example, minimizing current drain is desirable because the capacity of the power supply is limited. Any current saved while monitoring the communication channel when the receiver is not receiving a desired signal can be used to extend the time during which the receiver is receiving the desired signal. In a portable radiotelephone, for example, reduced current drain means the user enjoys longer talk time.
Although prior art portable receivers operating in discontinuous receive mode consume significantly less current drain from portable power supplies than receivers operating in continuous receiver mode, the desire to further extend service from portable receivers has caused the discontinuous receive mode of operation to be examined more closely. In a receiver operating in discontinuous receive mode, the required time for the receiver to turn on before the receiver is prepared to receive and decode signals on the communication channel, i.e., the "turn on time", is longer than the time taken for the receiver to turn off the receive and decode functions. The turn on time is longer because receiver circuitry and internal signals need be initialized and stabilized before a desirable signal can be received. The prior art has not addressed minimizing the turn on time of the receiver operating in discontinuous receive mode to further reduce the current drain on the power supply and thereby extend the time that the receiver could provide service.
Accordingly, there is a need for an apparatus and a method for minimizing the turn on time for a receiver operating in discontinuous receive mode to advantageously reduce current drain on the power supply thereby extending the time that the receiver could provide service.